3.7.5 - Back-up protection
There are times when the overload protection
has insufficient breaking capacity safely to interrupt the
prospective short circuit current at the point of the installation
where it is situated. An instance would be where a large
number of low-rated miniature circuit breakers each with
a breaking capacity of 3 kA are fed by a large cable. In
the event of a short circuit which gives a current of 8
kA, there is a good chance that the miniature circuit breaker
concerned will be unable to break the fault.
Perhaps the fault current may continue
to flow in the form of an arc across the opened circuit
breaker contacts, causing a very high temperature and the
danger of fire. Of course, if this happened the circuit
breaker would be destroyed.
Fig 3.24 Back-up protection
The normal method of protection
is to 'back up' the circuit with a protective device which
has the necessary breaking capacity. For example, the group
of miniature circuit breakers mentioned above could be backed
up by an HBC fuse as shown in (Fig 3.24}.
It will be appreciated that when a protective
device operates it does not do so instantaneously, and fault
current will flow through it to the circuit it seeks to
protect. The time for which such a current flows is a critical
factor in the damage that may be done to the system before
the fault clears. Damage applies to cables, switchgear,
protective devices, etc. if the fault is not cleared quickly
enough. This damage will be the result of the release of
the energy of the fault current, and the system designer
will aim to minimise it by calculation and by consulting
manufacturer's data of energy let-through.